Evidence blasted into space: Mystery why some meteorites look less shocked solved

Knowing what happens when meteorites collide is important for understanding the evolution of the solar system because it provides a window into the solar system's past. And so, planetary scientists as well as astrobiologists analyzing meteorite samples have been puzzled to find that meteorites containing carbon show much less evidence of high-speed impacts than those without. It is as if the ones containing carbon all somehow collided at lower speeds, although it is unclear why that should be. Kobe University astrophysicist KUROSAWA Kosuke says: "I specialize in impact physics and am interested in how the meteorite material changes in response to impacts, something called 'shock metamorphism.' And so I was very interested in this question."

Kurosawa was inspired by a theory put forward 20 years ago by another Kobe University researcher that the impact produces degassed vapor from water-containing minerals in the meteorite which then ejects the evidence into space. "I thought the idea was brilliant, but it had problems. For one, they did not perform calculations whether this process would produce enough water vapor. Also, there are carbon-containing meteorites without such water-containing minerals that also seem to be less shocked," explains the astrophysicist. Thinking that the carbon-containing materials themselves should behave differently when shocked, he decided to investigate this idea using a device he had developed: a two-stage light gas gun connected to a sample chamber. This setup allowed Kurosawa and his team to collect and analyze the gases produced by a pellet's high-speed impact into a sample that mimicked meteorites both with and without carbon without the measurements being contaminated by the gases produced by the gun shot itself.

The Kobe University team now published their results in the journal Nature Communications. Their experiments revealed that impacts on carbon-containing meteorites cause chemical reactions that produce extremely hot carbon monoxide and carbon dioxide gases. Kurosawa says: "We found that the momentum of the ensuing explosion is enough to eject the surrounding highly-shocked rock material into space. Such explosions occur on carbon-rich meteorites, but not on carbon-poor ones." The team thus concluded that carbon-containing meteorites are no less shocked, but that, in fact, the evidence is quite literally blown away.

All may not be lost, however. On larger space rocks such as the dwarf planet Ceres, the team calculated that gravity may be strong enough to pull the ejected material back to the body's surface. "Our results predict that Ceres should have accumulated highly-shocked material produced by these impacts, and so we believe that this provides a guideline for planning the next generation of planetary exploration missions," Kurosawa explains.

This research was funded by the Japan Society for the Promotion of Science (grant JP19H00726), the Hyogo Science and Technology Association (grant #6077), and the Science and Technology Facilities Council (grant ST/S000615/1). It was conducted in collaboration with researchers from the Chiba Institute of Technology and Imperial College London. This work was supported by ISAS/JAXA as a collaborative program with the Hypervelocity Impact Facility. Numerical computations and analyses were in part carried out on the general-purpose PC cluster and the analysis servers at Center for Computational Astrophysics, National Astronomical Observatory of Japan.